Streptococcus mutans, etiologic agent of dental caries, is divided into 5 serotypes. We have isolated and characterized antigens possessing determinants for serotypes a and b. The b antigen is a wall-localized glycerol teichoic acid and cross-reacts with heart-reactive antibody in human rheumatic sera. Rabbit anti-b sera contain antibodies that bind to human myocardial sections. The a antigen and anti-a sera do not display the above cross-reactive properties. These findings dictate caution in the use of anti-cariogenic vaccines, and suggest that special oral prophylactic procedures may be required for rheumatics. During the coming period, we intend to determine the ubiquity of the b GTA antigen. Many reports in the literature indicate anti-b sera cross-react strongly with certain other viridans and hemolytic streptococci. We plan to screen cultures for such activity, and isolate and characterize the common antigen by our described methods. Each purified antigen will be assayed for reactivity with human autoantibody, and homologous rabbit sera tested for possession of heart-reactive properties. The discovery of this unique, common antigen in many other streptococci may affect present modes of antibiotic therapy for rheumatics. We intend, also, to continue our study of the biochemical and immunochemical natures of cell-wall antigens that determine serotypes c and d. Each antigen will be extracted, purified, and characterized as to biochemical structure. The nature of haptenic determinants will be studied and related strains in each serotype examined for possession of common antigens. Ability to react with heart-reactive antibody also will be determined.